The cytogenetic radiation damage to lymphocytes after in-vitro labeling of mixed leukocytes and isolated lymphocytes with 99mTc-hexamethylpropyleneamine oxime (HMPAO) was evaluated using the cytokinesis-blocked micronucleus assay. A direct assessment of the radiation damage to the lymphocytes after a labeling procedure of leukocytes separated from 46 ml blood with 740 MBq of 99mTc-HMPAO was not possible due to an almost complete impairment of the proliferative capacity. By starting with isolated lymphocytes, the number of micronuclei was studied versus the intracellular activity concentration in the range 0-3 MBq/10(7) lymphocytes for three donors. A comparison of these results with the dose response of the micronucleus incidence in lymphocytes after in-vitro irradiation with x-rays allowed an individual assessment of the x-ray dose, inducing the equivalent amount of clastogenic damage as the intracellular activity after 99mTc-HMPAO labeling. Based on an extrapolation of these data, the radiation damage of the lymphocytes due to self-irradiation in a labeling procedure of leukocytes with 740 MBq of 99mTc-HMPAO was estimated to be equivalent to 26 Gy of x-rays. Due to the observed almost complete inhibition of the proliferative capacity at this high dose level, the increased risk for a lymphoid malignancy after administration of isolated lymphocytes or mixed leukocytes labeled with 99mTc-HMPAO activities sufficient for scintigraphy can be regarded as small.